The generation of a uniform corona field is vital for accomplishing the fullest utility of many modern electrophotographic mediums. Conventional corona charging includes corona electrode assemblies in the form of a fine wire suspended over the chargeable surface which is commonly plagued with performance nonuniformities. Most pronounced of these nonuniformities is the presence of "beads" or hot spots on the corona wire. These beads appear as points, on the wire, having increased current concentration, and are believed to be wrought by not only physical flaws in the wire surface, but also by irregular electric field concentrations. Therefore, it became clear sometime ago that impovement could be had, at least insofar as apparent net surface charge effect is concerned, if these beads could be moved about relative to the surface while the charging action is going on. In particular, Rosenthal describes apparatus which can perform such movement in U.S. Pat. No. 2,856,533 "Moving Wire Corona." The wire moves axially as a continuous, endless loop in one form; in another form it rotates about its axis; whilst in yet another form it oscillates to and fro along its axis. In each case the beads which may form are "blurred" and thus made to substantially disappear insofar as their effect on the success of good charge acceptance uniformity by the receptor member is concerned. Giaimo in U.S. Pat. No. 2,956,487 "Electrostatic Printing" gives illustration of a stationary multiwire corona 27 which may set together in plurality to offset some of the charge negating effects produced by any given orientation of heads on a lone wire. Jarvis in U.S. Pat. No. 3,233,156 "Electrostatic Charging Methods and Apparatus" shows a rotating wire wound as a spiral which can serve to sweep across a photoconductor surface. Also depicted is a rotating helix of discs, and a rotating "brush" arrangement of needle like electrodes. In each of these cases, mechanical rotation is believed to enhance the laid-down charge uniformity through bead dispersion, etc.
In each of the teachings of Rosenthal and Jarvis complex and difficult to fabricate and maintain mechanical devices are relied upon to resolve what is effectually an electric problem. With more sensitive photoconductor materials and improved imaging agents, it must be recognized that even the cyclic mechanical patterns set up by these earlier devices can be disturbing.
Giaimo does not introduce motion into the corona electrode structure. Rather he seeks charge distribution uniformity by providing several parallel corona electrodes. This introduces the alternative nonuniformities produced by electrostatic repulsion between the overlapping like-polarity corona fields. The attendant field nonuniformities offset any advantages sought by multiwire corona arrangements when working with sensitive electrophotographic materials.
Therefore, in view of these earlier approaches, the alert artisian will quickly realize that the manipulation of the electric fields, i.e., the sequential scanning of a number of corona electrodes, as herein taught can produce an effective corona "blanket," a continuous acting corona region, which has an average charging effect on a charge receptor (photoconductor) surface oriented adjacently which is of maximally uniform value. Furthermore, the artisian will appreciate that no mechanical esoterica is employed to attain the desired results, thereby enhancing the overall life expectancy of a using machine and generally reducing the cost to produce the machine.